The effects of contemporary increases in riverine freshwater into the Arctic Ocean are estimated from ocean model simulations, using two runoff data sets. One runoff data set which is based on older climatological data, which has no inter-annual variability after 2007 and as such does not represent the observed increases in river runoff into the Arctic. The other data set comes from a hydrological model developed for the Arctic drainage basin, which includes contemporary changes in the climate. In the pan-Arctic this new data set represents an approximately 11% increase in runoff, compared with the older climatological data. Comparing two ocean model runs forced with the different runoff data sets, overall changes in different freshwater markers across the basin were found to be between 5-10%, depending on the area investigated. The strongest increases were seen from the Siberian rivers, which in turn caused the strongest freshening in the Eastern Arctic.

Rising temperatures and an acceleration of the hydrological cycle due to climate change are increasing river discharge, causing permafrost thaw, glacial melt, and a shift to a groundwater-dominated system in the Arctic. These changes are funnelled to coastal regions of the Arctic Ocean where the implications for the distributions of nutrients and biogeochemical constituents are unclear. In this study, we investigate the impact of terrestrial runoff on marine biogeochemistry in Inuit Nunangat (the Canadian Arctic Archipelago) --- a key pathway for transport and modification of waters from the Arctic Ocean to the North Atlantic --- using sensitivity experiments from 2002-2020 with an ocean model of manganese (Mn). The micronutrient Mn traces terrestrial runoff and the modification of geochemical constituents of runoff during transit. The heterogeneity in Arctic runoff composition creates distinct terrestrial fingerprints of influence in the ocean: continental runoff influences Mn in the southwestern Archipelago, glacial runoff dominates the northeast, and their influence co-occurs in central Parry Channel. Glacial runoff carries micronutrients southward from Nares Strait in the late summer and may help support longer phytoplankton blooms in the Pikialasorsuaq polynya. Enhanced glacial runoff may increase micronutrients delivered downstream to Baffin Bay, accounting for up to 18% of dissolved Mn fluxes seasonally and 6% annually. These findings highlight how climate induced changes to terrestrial runoff may impact the geochemical composition of the marine environment, and will help to predict the extent of these impacts from ongoing alterations of the Arctic hydrological cycle.

Biogeochemical cycles in the Arctic Ocean are sensitive to the transport of materials from continental shelves into central basins by sea ice. However, it is difficult to assess the net effect of this supply mechanism due to the spatial heterogeneity of sea ice content. Manganese (Mn) is a micronutrient and tracer which integrates source fluctuations in space and time. The Arctic Ocean surface Mn maximum is attributed to freshwater, but studies struggle to distinguish sea ice and river contributions. Informed by observations from 2009 IPY and 2015 Canadian GEOTRACES cruises, we developed a three-dimensional dissolved Mn model within a 1/12 degree coupled ocean-ice model centered on the Canada Basin and the Canadian Arctic Archipelago (CAA). Simulations from 2002-2019 indicate that annually, 87-93% of Mn contributed to the Canada Basin upper ocean is released by sea ice, while rivers, although locally significant, contribute only 2.2-8.5%. Downstream, sea ice provides 34% of Mn transported from Parry Channel into Baffin Bay. While rivers are often considered the main source of Mn, our findings suggest that in the Canada Basin they are less important than sea ice. However, within the shelf-dominated CAA, both rivers and sediment resuspension are important. Climate induced disruption of the transpolar drift may reduce the Canada Basin Mn maximum and supply downstream. Other micronutrients found in sediments, such as Fe, may be similarly affected. These results highlight the vulnerability of the biogeochemical supply mechanisms in the Arctic Ocean and the subpolar seas to climatic changes.

Analyzing a high-resolution (1/60°) numerical model over 2008 to 2018, the inter-annual variability of the West Greenland Coastal Current (WGCC) on the shelf and West Greenland Current (WGC) at shelf break is presented. Both currents flow from Cape Farewell and extend to Davis Strait, with their model speeds and transports corresponding well with observations. The inter-annual variability of the WGCC and WGC near southwest Greenland are opposite, with the former declining while the latter strengthened, both by a speed change above 0.1 m/s. Both currents are predominantly buoyancy forced, but wind forcing becomes more dominant towards Davis Strait. The main exchanges from the two currents to interior occur between Cape Desolation and Fylla Bank, with net volume, freshwater, heat transport decreases of 1.4 Sv, 13 mSv, 36.7 TW. The freshwater transport of the WGC itself does not drop in between these sections, receiving freshwater from the WGCC to compensate for the losses to the basin interior. Thus, we see significant freshwater (83.1 mSv) and heat transports (70.7 TW) of the WGC remaining at Fylla Bank that reach the northern basin instead of being fluxed into the interior pof the Labrador Sea. This suggests that the exchange between the current system and the interior is more limited than previously thought, and most of the Greenland and Arctic melt reaches the northern Labrador Sea. Our results highlight the importance of resolving the WGCC and shelf processes.

As the sea-ice modeling community is shifting to advanced numerical frameworks, developing new sea-ice rheologies, and increasing model spatial resolution, ubiquitous deformation features in the Arctic sea ice are now being resolved by sea-ice models. Initiated at the Forum for Arctic Modelling and Observational Synthesis (FAMOS), the Sea Ice Rheology Experiment (SIREx) aims at evaluating current state-of-the-art sea-ice models using existing and new metrics to understand how the simulated deformation fields are affected by different representations of sea-ice physics (rheology) and by model configuration. Part I of the SIREx analysis is concerned with evaluation of the statistical distribution and scaling properties of sea-ice deformation fields from 35 different simulations against those from the RADARSAT Geophysical Processor System (RGPS). For the first time, the Viscous-Plastic (and the Elastic-Viscous-Plastic variant), Elastic-Anisotropic-Plastic, and Maxwell-Elasto-Brittle rheologies are compared in a single study. We find that both plastic and brittle sea-ice rheologies have the potential to reproduce the observed RGPS deformation statistics, including multi-fractality. Model configuration (e.g. numerical convergence, atmospheric forcing, spatial resolution) and physical parameterizations (e.g. ice strength parameters and ice thickness distribution) both have effects as important as the choice of sea-ice rheology on the deformation statistics. It is therefore not straightforward to attribute model performance to a specific rheological framework using current deformation metrics. In light of these results, we further evaluate the statistical properties of simulated Linear Kinematic Features (LKFs) in a SIREx Part II companion paper.

Simulating sea-ice drift and deformation in the Arctic Ocean is still a challenge because of the multi-scale interaction of sea-ice floes that compose the Arctic sea ice cover. The Sea Ice Rheology Experiment (SIREx) is a model intercomparison project formed within the Forum of Arctic Modeling and Observational Synthesis (FAMOS) to collect and design skill metrics to evaluate different recently suggested approaches for modeling linear kinematic features (LKFs) and provide guidance for modeling small-scale deformation. In this contribution, spatial and temporal properties of LKFs are assessed in 36 simulations of state-of-the-art sea ice models and compared to deformation features derived from RADARSAT Geophysical Processor System (RGPS). All simulations produce LKFs, but only very few models realistically simulate at least some statistics of LKF properties such as densities, lengths, or growth rates. All SIREx models overestimate the angle of fracture between conjugate pairs of LKFs and LKF lifetimes pointing to inaccurate model physics. The temporal and spatial resolution of a simulation and the spatial resolution of atmospheric forcing affect simulated LKFs as much as the model’s sea ice rheology and numerics. Only in very high resolution simulations (≤2\,km) the concentration and thickness anomalies along LKFs are large enough to affect air-ice-ocean interaction processes.

The thermohaline intrusion of the warm and saline Atlantic Water (AW) into the Arctic Ocean, referred to as “Arctic Atlantification”, has significant implications and feedback to the dynamics and thermodynamics of the Arctic Ocean. The AW enters the Arctic Ocean through two gateways: Fram Strait and the Barents Sea Opening (BSO). The relative strength of these two AW branches dominates the oceanic heat contribution to the Arctic Ocean. In conjunction with the measurements in key hydrographic sections, numerical ocean modelling provides us with a useful tool to characterize and corroborate the temporal and spatial variability of the AW branches. Simulations are conducted using the regional configuration Arctic and North Hemispheric Atlantic (ANHA) of the ocean/sea-ice model NEMO running at 1/4° and 1/12° resolution. Online passive tracers from the model configuration are used to trace the pathways of the AW inflow in the Arctic Ocean. With the AW becoming more important to the dynamics of the Arctic Ocean, this study aims to examine its variability, transformation, impacts, and ultimately track how it evolves. While the heat in the Fram Strait Branch Water (FSBW) dissipates in a slower process through the mixing with the ambient cold water below sea surface, the vast majority of the heat loss of the Barents Sea Branch Water (BSBW) takes place in Barents Sea due to the sea surface cooling, leading to Cold Atlantic Water (CAW) production during 2013 and 2014. The CAW pulses result in a significant heat content reduction in the eastern Arctic.